This revised renewal application represents a continuation of research we have pursued for the past 12 years on the Molecular Pathology of Chronic Lung Disease. The work has focused primarily on the mechanisms by which complement activation and generation of the anaphylatoxins, C3a and C5a, function in innate and adaptive immunity, and critically modulate the pathophysiologic responses to lung injury. The previous funding cycle witnessed several exciting developments including the observation that C3a receptor deficient mice are protected from development of allergic airway hyperresponsiveness, coupled with preliminary data that expression of this protein on smooth muscle cells can restore the wild type phenotype. We have recently demonstrated protection from the airway hyperresponsiveness associated with RSV infection in C3aR-/- mice. Additional experiments showed that mice deficient in the C5a receptor are protected from edema and influx of inflammatory cells in contact sensitivity reactions, revealing the dependence upon complement activation of this response. More recent investigations have focused on the role of second identified receptor for the C5a anaphylatoxin, C5L2. Mice with targeted deletion of this molecule exhibit dramatically enhanced C5a/C5aR mediated inflammation, and we hypothesize that C5L2 is an intrinsic negative regulator of complement. Studies presented here reveal C5L2 is an intracellular receptor in human neutrophils. Mechanistically C5L2 is activated following binding of C5a to the C5aR. Ligand bound C5aR is then internalized and co-localizes with C5L2 and -arrestin. The C5L2--arrestin complex inhibits C5a mediated ERK1/2 activation, thereby negatively modulating C5a-C5aR chemotaxis. In order to further our goals of understanding the role of the complement anaphylatoxins in lung injury, and explore potentials for therapy, we will determine the mechanism(s) by which the intracellular C5a receptor, C5L2 negatively modulates the actions of C5a/C5adesArg (Aim 1). We will determine the molecular basis of the C5L2-C5aR--arrestin interactions. We will determine the agonist specificity of C5L2 mediated modulation of neutrophil functions. Further, we will determine the role of the PDZ binding motif on C5L2. Studies described for Aim 2 will test the hypothesis that C3a/C3aR interactions modulate the effector phases of allergic lung injury. We will determine the role of C3aR expression on airway smooth muscle cells in the manifestation of allergic airway hyperresponsiveness. In Aim 3, we will test the hypothesis that the role of the eosinophil in the initiation of allergic airway disease is under genetic control. We will map quantitative trait loci (QTLs) and potential candidate genes regulating the contribution of eosinophils to airway hyperresponsiveness and generation of a Th2 resonse by crossing ?dblGATA knockout mice on the C5lBL/6 and Balb/c backgrounds.Taken together, the results of these continuing investigations have promise for evaluating potential targets of therapy in human lung disease.